CN1842712A - Impedance measuring circuit, its method, and capacitance measuring circuit - Google Patents
Impedance measuring circuit, its method, and capacitance measuring circuit Download PDFInfo
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- H—ELECTRICITY
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- H04R29/004—Monitoring arrangements; Testing arrangements for microphones
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- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
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Abstract
A capacitance measuring circuit 10 comprising a DC voltage generator 11, an operational amplifier 14 having a noninverted input terminal connected to a predetermined potential, an impedance converter 16, a resistor R1 12 connected between the DC voltage generator 11 and the inverted input terminal of the operational amplifier 14, a resistor R2 13 connected between the inverted output terminal of the operational amplifier 14 and the output terminal of the impedance converter 16, and a capacitor 15 connected between the output terminal of the operational amplifier 14 and the input terminal of the impedance converter 16, wherein a capacitor 17 to be measured is connected between the input terminal of the impedance converter 16 and a predetermined potential.
Description
Technical Field
The present invention relates to a circuit and a method for detecting impedance and capacitance, and more particularly, to a circuit and a method for detecting minute impedance and capacitance with high accuracy.
Background
As a conventional example of the capacitance detection circuit, the one described in Japanese patent laid-open No. 9-280806 can be cited. Fig. 1 is a circuit diagram of the capacitance detection circuit. In the detection circuit, a capacitance sensor 92 formed of electrodes 90 and 91 is connected to an inverting input terminal of an operational amplifier 95 via a signal line 93. A capacitor 96 is connected between the output terminal and the inverting input terminal of the operational amplifier 95, and an ac voltage Vac is applied to the non-inverting input terminal. Further, the signal line 93 is covered with a shield wire 94, thereby electrically shielding the interference noise. And the shielded line 94 is connected to the non-inverting input terminal of the operational amplifier 95. The output voltage Vd is output from the output terminal of the operational amplifier 95 via the transformer 97.
In the detection circuit, the inverting input terminal and the non-inverting input terminal of the operational amplifier 95 are in a virtual short circuit state, and the signal line 93 connected to the inverting input terminal and the shield line 94 connected to the non-inverting input terminal are at substantially the same potential. Thus, the signal line 93 is protected by the shield line 94, that is, parasitic capacitance between the signal line 93 and the shield line 94 can be eliminated, and the output voltage Vd which is less susceptible to the parasitic capacitance can be obtained.
However, according to the above-described conventional technique, when the capacitance of the capacitance sensor 92 is large to some extent, although the accurate output voltage Vd which is less likely to be affected by the parasitic capacitance between the signal line 93 and the shield line 94 can be obtained, a large error is caused when a small capacitance of the order of several pF or fF (femtofarad) or less is detected.
Further, a tracking error or the like occurs in the operational amplifier 95 depending on the frequency of the applied ac voltage Vac, and thus a slight phase or amplitude shift is finally generated between the voltages of the inverting input terminal and the non-inverting input terminal in the virtual short circuit state, and a detection error becomes large.
On the other hand, in a portable and small-sized voice communication apparatus represented by a mobile phone or the like, an integrated amplifier circuit is required to convert sound detected by a capacitance sensor such as a capacitance microphone into an electric signal with high sensitivity and faithfully. If a minute capacitance of several pF or an fF level or a change thereof can be accurately detected, a high-performance microphone capable of faithfully detecting a sound with extremely high sensitivity can be realized, and the sound pickup performance of a voice communication apparatus such as a mobile phone can be improved dramatically.
Disclosure of Invention
In view of the above circumstances, it is an object of the present invention to provide an impedance detection circuit and a capacitance detection circuit which can accurately detect a minute capacitance and which are suitable for impedance detection mainly using a capacitance sensor such as a condenser microphone used in a portable and small-sized voice communication device.
To achieve the above object, an impedance detection circuit and a capacitance detection circuit according to the present invention are an impedance detection circuit that outputs a detection signal corresponding to an impedance of an impedance to be detected, including: an impedance converter having a high input impedance and a low output impedance; a capacitive first impedance element; a first operational amplifier; a dc voltage generator for applying a dc voltage to the first operational amplifier; a signal output terminal connected to an output of the first operational amplifier; wherein one end of the impedance to be measured and one end of the first impedance element are connected to an input terminal of the impedance converter, the first impedance element and the impedance converter are included in a negative feedback loop of the first operational amplifier, and the impedance to be measured is provided adjacent to the impedance detection circuit.
Further, an impedance detection circuit and a capacitance detection circuit of the present invention are an impedance detection circuit that outputs a detection signal corresponding to an impedance of an impedance to be detected, and the impedance detection circuit includes: an impedance converter having a high input impedance and a low output impedance; a capacitive first impedance element; a first operational amplifier; a dc voltage generator for applying a dc voltage to the first operational amplifier; a signal output terminal connected to an output of the first operational amplifier; wherein one end of the impedance to be measured and one end of the first impedance element are connected to an input terminal of the impedance converter, the first impedance element and the impedance converter are included in a negative feedback loop of the first operational amplifier, and the impedance to be measured is provided in proximity to the first impedance element and the impedance converter.
Here, in the present specification, "close" means that the parasitic capacitance of the signal line is in a state of not more than 10 times larger than the capacitance value of the capacitor to be measured or the capacitance value of the capacitive first impedance element. This is because the capacitance detection circuit of the present invention can prevent a large deterioration in detection sensitivity when the parasitic capacitance of the signal line is a capacitance value of a last order of magnitude not exceeding the capacitance value of the connected element, which is empirically derived. The parasitic capacitance of the signal line can be detected by detecting the capacitance in a state where the capacitor to be detected, the first impedance element, and the impedance converter are not connected to the signal line. In the present specification, a state in which adjacent connection is performed under the above-described proximity condition is referred to as "adjacent".
Further, an impedance detection method and a capacitance detection method according to the present invention are an impedance detection method for outputting a detection signal corresponding to an impedance change (such as a change in capacitance) of an impedance to be detected, wherein a capacitive first impedance element is connected between an output terminal of an operational amplifier and an input terminal of an impedance converter, the impedance to be detected is connected between the input terminal of the impedance converter and a predetermined potential, a dc voltage is applied to an inverting input terminal of the operational amplifier via a resistor, the other input terminal is set to the predetermined potential, a voltage appearing at the output terminal of the operational amplifier is outputted as the detection signal, and the impedance to be detected is connected in proximity to the impedance converter and the first impedance element.
As a specific example, a capacitance detection circuit is configured, the circuit including: a DC voltage generator; an operational amplifier having a non-inverting input terminal connected to a predetermined potential; an impedance transformer; a resistor (R2) connected between the inverting input terminal of the operational amplifier and the output terminal of the impedance converter; a capacitive impedance element connected between an output terminal of the operational amplifier and an input terminal of the impedance converter; the capacitor to be measured is connected between the input terminal of the impedance converter and a predetermined potential, and is disposed at a position adjacent to the capacitance detection circuit or close to the capacitance detection circuit by a short distance such that the parasitic capacitance of the signal line does not exceed 10 times the maximum capacitance value of the connected element. Here, the predetermined potential refers to any one of a reference potential, a predetermined dc potential, a ground potential, or a floating state, and an optimal one may be selected according to the embodiment. In addition, a resistor (R) connected between the DC voltage generator and the inverting input terminal of the operational amplifier may be added1)。
According to the above configuration, while a constant voltage is applied to the impedance to be measured, almost all of the current flowing through the impedance to be measured flows to the impedance element, and thus, a signal corresponding to the impedance of the impedance to be measured is output from the signal output terminal.
In addition, a resistor may be connected in parallel with the impedance element.
Further, it is also possible to connect one end of the impedance to be measured and the input terminal of the impedance converter through a signal line covered with a shielding member, and add a protective voltage applying member for applying a given voltage to the shielding member. Here, the predetermined potential is any constant potential, preferably ground, but may be the same potential as the voltage of the signal line. The circuit operation is stabilized by applying a given voltage to the shielding member.
The protective voltage applying member is, for example, a member for connecting the shielding member to the ground, and generates a predetermined voltage by using the output voltage of the dc voltage generator or the output voltage of the impedance converter as an input.
The impedance converter may be constituted by a voltage follower, or may be constituted by a voltage amplifier circuit having a voltage gain smaller than 1 or larger than 1. In addition, if the input stages of these impedance converters are made as circuits composed of MOSFETs, the input impedance can be further increased.
Further, as an application of the present invention, it is possible to use a capacitive sensor in which a resistance to be measured is set to detect a physical quantity from a change in capacitance, and a capacitance detection circuit as an impedance detection circuit is formed on a printed wiring board or a silicon substrate, and these capacitive sensors are fixed to the substrate, preferably integrally formed. More specifically, a condenser microphone is used as a resistance to be measured, a capacitance detection circuit is realized by an IC, and the condenser microphone is integrated with the IC, and may be placed in a case (shield case) as a microphone used in a portable telephone set or the like. In this case, the condenser microphone and the IC are fixed at adjacent positions and connected to each other by using a conductive plate, a wiring pattern, a bonding wire, or the like.
Drawings
Fig. 1 is a circuit diagram of a conventional capacitance detection circuit.
Fig. 2 is a circuit diagram of a capacitance detection circuit in the first embodiment of the present invention.
Fig. 3 is a specific circuit example of an impedance converter in the capacitance detection circuit shown in fig. 2.
Fig. 4 is a circuit diagram of a capacitance detection circuit in a second embodiment of the present invention.
Fig. 5 is a circuit diagram of a modification of the capacitance detection circuit shown in fig. 4.
Fig. 6 is a specific circuit example of the protection voltage applying circuit shown in fig. 4 and 5.
Fig. 7 is a schematic diagram (a sectional view of a microphone) of applying the capacitance detection circuit of the present invention to an electronic device.
Fig. 8 is a schematic external view of the microphone shown in fig. 7, in which fig. 8A is a plan view, fig. 8B is a front view, and fig. 8C is a bottom view.
Fig. 9 is a cross-sectional view of another example of a microphone.
Fig. 10 is a schematic external view of the microphone shown in fig. 9, in which fig. 10A is a plan view and fig. 10B is a front view.
Fig. 11 is a circuit diagram of a capacitance detection circuit in another embodiment of the invention.
Detailed Description
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
(first embodiment)
Fig. 2 is a circuit diagram of a capacitance detection circuit in the first embodiment of the present invention. In the present figure, a capacitor under test 17 (a capacitance sensor such as a capacitance microphone that detects various physical quantities by a change in capacitance Cs) as an impedance under test is connected to the capacitance detection circuit as an impedance detection circuit.
The capacitance detection circuit 10 is configured by a dc voltage generator 11 that generates a dc voltage, a resistor (R1)12, a resistor (R2)13, an operational amplifier 14, an impedance element (here, a capacitor having a capacitance Cf) 15, and an impedance converter 16, and outputs a detection signal (voltage Vout) corresponding to a temporal change in capacitance of a capacitor 17 to be measured from a signal output terminal 20. Here, "temporal change" includes changes in frequency, changes in pulse, changes gradually, and the like, and changes randomly in time, and thus, the periodicity is not always necessary.
A dc voltage generator 11, one end of which is connected to a given potential (in the present embodiment, ground), and generates a constant dc voltage from the other end (output terminal). A resistor (R1)12 is connected between the output terminal of the dc voltage generator 11 and the inverting input terminal of the operational amplifier 14. The operational amplifier 14 is a voltage amplifier whose input impedance and open loop gain are extremely high, where the non-inverting input terminal is connected to a given potential (in the present embodiment, ground), and the non-inverting input terminal and the inverting input terminal are in a virtual short state. A capacitor 15, an impedance converter 16, and a resistor (R2)13 are connected in series in this order to the negative feedback loop of the operational amplifier 14, that is, from the output terminal to the inverting input terminal of the operational amplifier 14.
The impedance converter 16 is a voltage amplifier having a very high input impedance, a very low output impedance, and a voltage gain of a times. One end of the capacitor 17 to be measured is connected to the input terminal 21 of the impedance converter 16, and the other end of the capacitor 17 to be measured is connected to a predetermined potential (in the present embodiment, ground). A signal output terminal 20 is connected to an output terminal of the operational amplifier 14, and outputs an output signal of the capacitance detection circuit 10, that is, a detection signal corresponding to a change in capacitance value of the capacitor 17 to be measured. In the present application, the variable a appearing as a times or the like represents an arbitrary real number other than zero (0).
The operation of the capacitance detection circuit 10 configured as described above is as follows.
In the inverting amplifier circuit including the resistor (R1)12, the resistor (R2)13, the operational amplifier 14, and the like, the two input terminals of the operational amplifier 14 are in a virtual short circuit state and are at the same potential (for example, 0V), and since the input impedance thereof is extremely high and no current flows, the current flowing through the resistor (R1)12 is Vin/R1, and the current flows entirely through the resistor (R2)13, and therefore, when the output voltage of the impedance converter 16 is set to V2, the following equation holds:
Vin/R1＝-V2/R2
by sorting the above, it can be seen that the output voltage V2 of the impedance converter 16 is:
v2 ═ - (R2/R1) · Vin (formula 1)
Since the voltage gain of the impedance converter 16 is a, the input voltage V1 is, in accordance with the relationship between the input voltage (voltage at the input terminal 21) V1 and the output voltage (voltage at the output terminal 22) V2:
v1 ═ - (1/a) · V2 (formula 2)
However, when the measured capacitor 17 is a condenser microphone or the like, its capacitance Cs changes according to the frequency of the inputted sound. Here, when the charge corresponding to the change flowing from the operational amplifier 14 to the capacitor 15, that is, from the capacitor 15 to the capacitor under test 17 is Δ Q (that is, the capacitance change component of the capacitor under test 17), since the input impedance of the impedance converter 16 is extremely high, the charge Q flows entirely to the capacitor under test 17, so that V1 becomes Δ Q/Δ Cs, and the change component Δ Vout of the voltage Vout of the detection signal output from the signal output terminal 20 is:
Δ vout ═ V1 (Δ Cs/Cf) (equation 3)
Elimination of V2 from the above equations 1 and 2 yields:
v1 ═ - (R2/R1) · (Vin/a) (formula 4)
Substituting V1 into equation 4 above, then:
Δvout＝-(1/Cf)·(R2/R1)·(Vin/A)·ΔCs
k Δ Cs (formula 5)
Wherein,
k- (1/Cf) · (R2/R1) · (Vin/A) (equation 6)
That is, the change component Δ Vout of the output voltage Vout of the detection signal is a value proportional to the change component Δ Cs of the capacitance Cs of the capacitor under test 17. Therefore, by extracting only the ac component Δ Vout of the detection signal output from the capacitance detection circuit 10, a signal corresponding to the sound input to the condenser microphone can be obtained. In this way, a valid signal (voltage corresponding to Δ Cs) corresponding to sound can be amplified effectively, so that a microphone with high sensitivity can be realized.
In addition, the scaling factor k shown in the above equation 6 is a constant value that does not include a term related to frequency (frequency of sound). Therefore, the capacitance detection circuit 10 outputs a faithful voltage signal corresponding to the intensity of sound with a constant gain, regardless of the frequency of sound. Here, the operation of the capacitor 17 to be measured is discussed in terms of voltage. On the other hand, to facilitate understanding, the analysis is performed from the viewpoint of current.
Now, the capacitance of the capacitor 17 to be measured is set to change in time as follows.
Cs ═ Cd + Δ Csin ω ct (equation 7)
At this time, Cd is the original reference capacitance of the capacitor 17, Δ C is the peak value of the change, and ω C is the frequency of change in the capacitance being detected of the capacitor 17. At this time, the current flowing through the measured capacitor 17 is:
Since the above-mentioned current flows entirely through the capacitor 15, therefore,
Here, since the currents of the equations 8 and 9 are equal to each other, the currents are equal to each other
Δ vout ═ - (Δ C · sin ω ct/Cf) · V1 (equation 10)
According to the formula 1 and the formula 2, the formula 10 can be expressed as:
Δvout＝-(ΔC·sinωct/Cf)·(Vin/A)
(R2/R1) (equation 11)
Also, the output is a variation component of the measured capacitor 17.
Further, since the capacitance detection circuit 10 operates according to DC driving (the DC voltage generator 11), noise interference and the like can be suppressed compared to AC driving, and stable operation can be achieved. Further, since components such as an ac transmitter are not required, the circuit scale can be reduced.
Fig. 3 shows a specific circuit example of the impedance converter 16 in the capacitance detection circuit 10 shown in fig. 2. Fig. 3A shows a voltage follower 16a using an operational amplifier 30. The inverting input terminal and the output terminal of the operational amplifier 30 are short-circuited. By setting the non-inverting input terminal of the operational amplifier 30 as the input of the impedance converter 16 and the output terminal of the operational amplifier 30 as the output of the impedance converter 16, the impedance converter 16 having a very high input impedance and a voltage gain a of 1 can be obtained.
Fig. 3B shows the in-phase amplifier circuit 16B using the operational amplifier 31. A resistor (R3)32 is connected between the inverting input terminal of the operational amplifier 31 and a predetermined potential, and a feedback resistor (R4)33) is connected between the inverting input terminal and the output terminal of the operational amplifier 31. By setting the non-inverting input terminal of the operational amplifier 31 as the input of the impedance converter 16 and the output terminal of the operational amplifier 31 as the output of the impedance converter 16, the impedance converter 16 having an extremely high input impedance and a voltage gain a of (R3+ R4)/R3 can be obtained.
Fig. 3C shows a circuit 16C in which a buffer of a CMOS structure is added to the input stage of the operational amplifier shown in fig. 3A or 3B. As shown, an N-type MOSFET34 and a P-type MOSFET35 are connected in series between the positive and negative power supplies via a resistor, and the output of the buffer is connected to the input of the operational amplifier 30 (or 31). By setting the input of the buffer as the input of the impedance converter 16 and the output terminal of the operational amplifier as the output of the impedance converter 16, the impedance converter 16 having an extremely high input impedance can be obtained.
Fig. 3D shows a circuit 16D such as a buffer of the input stage of fig. 3C. As shown, an N-type MOSFET34 and a P-type MOSFET35 are connected in series between positive and negative power supplies, and output from a connection portion of the two MOSFETs.
In fig. 3E, the non-inverting input of the operational amplifier 32 is used as the input of the impedance converter, and the output and the inverting input of the operational amplifier 32 are connected by a resistor. As shown in fig. 3D and 3E, the impedance converter 16 having an extremely high input impedance can be obtained by the above configuration.
According to the experiment of the present invention, in the impedance detection circuit of fig. 2, for example, when the primary capacitance of Cs is 20pF, if the parasitic capacitance of the signal line exceeds 200pF, the detection sensitivity is significantly deteriorated. In addition, the Cs was confirmed with several other capacitance values, and the same trend was also obtained.
In the circuit, the capacitance Cf as the first impedance element and the capacitor Cs to be measured are both capacitance elements connected to the signal line, and the same result as described above is obtained computationally for either element.
From these experimental results and experience, it is found that good detection sensitivity can be obtained if the capacitor to be measured is connected in close proximity to the first impedance element and the impedance transformer so that the parasitic capacitance of the signal line does not exceed a value of one order or more of the capacitance value of Cs or Cf.
(second embodiment)
Next, an impedance detection circuit in a second embodiment of the present invention will be described.
Fig. 4 is a circuit diagram of a capacitance detection circuit 40 as an impedance detection circuit in the second embodiment. The capacitance detection circuit 40 is equivalent to the capacitance detection circuit 10 in the first embodiment to which a protection function is added. That is, as a cable for connecting the capacitor under test 17 and the capacitance detection circuit 40, a signal line 41 (coaxial cable) covered with a shield line 42 is used, and a protection voltage application circuit 43a for applying a protection voltage having the same potential as the signal line 41 to the shield line 42 of the coaxial cable is added.
The protection voltage applying circuit 43a is connected between the output terminal of the dc voltage generator 11 and the shield line 42, amplifies (or divides) the output voltage Vin of the dc voltage generator 11 with a predetermined constant voltage gain using the input voltage Vin, generates a protection voltage having the same potential as the voltage of the signal line 41, and outputs the protection voltage to the shield line 42. In addition, the voltage gain of the protection voltage applying circuit 43a is V1/Vin, which is known from the above equation 4, and is adjusted to (-R2/R1) · (1/A).
With the above configuration, the signal line 41 and the shield line 42 can be kept at the same potential at all times, and thus the capacitance (parasitic capacitance) between them is eliminated, and thus measurement errors due to the parasitic capacitance added to the capacitance of the capacitor 17 to be measured can be avoided, and at the same time, the shield line 42 can be used to shield the noise of the signal line 41 from interfering with each other, thereby enabling more accurate and stable capacitance detection.
The connection position of the protection voltage applying circuit 43a for applying the protection voltage to the shield line 42 is not limited to the position between the dc voltage generator 11 and the shield line 42 shown in fig. 4, and for example, a capacitance detecting circuit 45 shown in fig. 5 may be provided between the output terminal of the impedance converter 16 and the shield line 42. At this time, the protection voltage applying circuit 43b (or 43c) may be adjusted as follows: the output voltage V2 of the impedance transformer 16 is input, and a protection voltage V1 is generated by amplification with a constant voltage gain (1/a) and applied to the shield line 42.
However, if the protection voltage application circuit is limited to DC application, the parasitic capacitance cannot be eliminated as expected, and therefore, in this case, it is most effective to have a simple structure and interfere with a ground connection that is difficult to enter.
Fig. 6 shows an example of a specific circuit of the protection voltage applying circuits 43a to c shown in fig. 4 or 5. The protection voltage application circuit 43a shown in fig. 6A is an inverting amplifier circuit having a variable resistor as a feedback resistor. By adjusting the resistance value of the feedback resistor, the voltage gain described above can be obtained, and thus a protection voltage having the same potential as the signal line 41 can be generated. The protection voltage application circuit 43B shown in fig. 6B is a non-inverting amplifier circuit including two resistors and one operational amplifier. The protection voltage applying circuit 43C shown in fig. 6C is a voltage follower including two resistors and one operational amplifier. In fig. 6B and 6C, a protection voltage having the same potential as that of the signal line 41 in fig. 5 may be generated by adjusting the resistance value or the like.
In addition, when an arithmetic error, a tracking error, or the like occurs, it is possible to reduce the error when the gain a is set to 1, and therefore, it is preferable to set a to 1.
As an application of the capacitance detection circuit of the present invention to an electronic device, a sensor for detecting a physical quantity according to a change in impedance can be used as a resistance to be detected, an impedance detection circuit can be formed on a printed wiring board or a silicon substrate, and an integrated structure for fixing the sensor and the substrate can be adopted. More specifically, a condenser microphone is used as an impedance to be measured, a capacitance detection circuit is realized by an IC, and the condenser microphone is integrated with the IC while being placed in a case (shield case) as a microphone used in a portable telephone set or the like.
Fig. 7 shows an application example of the capacitance detection circuit in the first embodiment described above in an electronic device. Here, a cross-sectional view of a microphone 50 in which a condenser microphone and a capacitance detection circuit are integrated is shown, the microphone 50 being used in a portable telephone or the like. The microphone 50 is composed of: a cover 51 having a sound hole 52; a diaphragm 53 that can vibrate according to sound; a ring 54 for fixing the diaphragm 53; a partition plate 55 a; a fixed electrode 56 disposed opposite to the diaphragm 53 via a spacer 55 a; an insulating plate 55b for supporting the fixed electrode 56; an IC chip 58 formed with the capacitance detection circuit in the above embodiment and fixed to the back surface of the insulating plate 55 b; an IC package 59 in which an IC chip 58 is molded; the external electrodes 61a and 61b are connected to the IC chip 58 through bonding wires, contact holes, and the like.
The diaphragm 53, which is one electrode forming a capacitor, is connected to a predetermined potential (in the present embodiment, to ground), and the fixed electrode 56, which is the other electrode, is connected to a circuit of the IC chip 58 through an aluminum plate, a bonding wire, a contact hole, or the like. The capacitance of the capacitor formed by the diaphragm 53 and the fixed electrode 56, or a change in the capacitance, is detected by a capacitance detection circuit in the IC chip 58 adjacent to each other through the insulating plate 55b, converted into an electric signal, and output from the external electrodes 61a and 61 b. In addition, the lid 51 is made of metal such as aluminum, and the lid 51 and a conductive film (not shown) formed on the upper surface of the insulating substrate 60 function as a shield case for shielding the capacitors 53 and 56 or the IC chip 58 inside noise. In the present embodiment, the fixed electrode 56 is connected to the circuit and the diaphragm 53 is connected to a predetermined potential, but the diaphragm may be connected to the circuit and the fixed electrode 56 may be connected to a predetermined potential. Wherein the former is better empirically.
Fig. 8 is a schematic external view of the microphone 50 shown in fig. 7. Fig. 8A is a plan view, fig. 8B is a front view, and fig. 8C is a bottom view. The size of the lid 51 shown in fig. 8A and 8B is, for example, approximately 5mm × 2mm in height. The four external electrodes 61a to 61d shown in fig. 8C are, for example, two terminals for power supply and two terminals for output signal of the capacitance detection circuit.
In the example of application, the capacitor under test (here the condenser microphone) and the capacitance detection circuit (here the IC chip) are arranged adjacent in the said close condition and are connected by an electrical conductor of very short length. These elements are covered with a shielding member such as a metallic cover. Therefore, in the application example, an adverse effect of disturbance noise or the like incorporated in a signal line (conductor) for connecting the capacitor to be tested and the capacitance detection circuit can be ignored.
In the present application example, it is preferable that the capacitor to be measured and the capacitance detection circuit are connected by a shortest route through an unshielded (unshielded) conductive plate, a wiring pattern, a bonding wire, a wire, or the like. That is, since the present application example is a small microphone having no shield member on the signal line, the capacitor to be measured and the capacitance detection circuit are connected by an extremely short conductor, and a special circuit for applying a protection voltage to a shield line or the like is not required, so that the circuit scale is not enlarged, and the integration of the circuit is not affected.
As another example of the microphone, as shown in fig. 9 and 10, a circuit is loaded on a substrate. The structure is substantially the same except that the capacitance detecting circuit in fig. 7 is loaded on the substrate 62.
Further, when the second embodiment is adopted in the present application example, the circuit scale will be slightly enlarged due to the portion relating to the shielding of the signal line, but the scheme is more effective from the viewpoint of achieving detection with higher accuracy, and therefore the structure can also be used.
In the above, the capacitance detection circuit of the present invention has been described based on two embodiments and application examples of products, but the present invention is not limited to these embodiments and application examples.
For example, in the second embodiment, as the cable for connecting the capacitor under test 17 and the capacitance detection circuit 40, a single-layer cable is used, but a double-layer cable may be used instead. In this case, by applying a protective voltage to the inner shield layer covering the signal line and connecting the outer shield layer covering the inner shield layer to a predetermined potential or ground, the shielding effect against the interference noise can be improved.
Further, as shown in fig. 11, a resistor 18 may be added so as to be connected in parallel with the capacitor 15 in the capacitance detection circuits 10 and 30 in the above-described embodiments. Thus, the connection point between the capacitor 15 and the capacitor under test 17 is connected to the output terminal of the first operational amplifier 14 via the resistor 18, so that the floating state in the form of a direct current is eliminated and the potential is fixed.
Furthermore, the connectable impedance to be measured is not limited to the condenser microphone, and may include all of the following devices for detecting various physical quantities: that is, acceleration sensors, seismometers, pressure sensors, displacement sensors, proximity sensors, contact sensors, ion sensors, humidity sensors, raindrop sensors, snow sensors, thunder sensors, placement sensors (displacement sensors), contact failure sensors, shape sensors, end point detection sensors, vibration sensors, ultrasonic sensors, angular velocity sensors, liquid amount sensors, gas sensors, infrared sensors, radiation sensors, water level meters, refrigeration sensors, moisture meters, vibration meters, charge sensors, printed circuit board detectors, and the like.
As is apparent from the above description, the capacitance detection circuit, the capacitance detection device, and the methods of the present invention detect the impedance of the impedance to be measured by applying a dc voltage to the operational amplifier and connecting the impedance to be measured to the signal line. That is, a capacitor is connected between the output terminal of the operational amplifier having the non-inverting input terminal connected to a predetermined potential and the input terminal of the impedance converter, and a measured impedance is connected between the input terminal of the impedance converter and the predetermined potential.
Thus, the entire current flowing through the impedance to be measured flows to the impedance element, and an accurate signal corresponding to the impedance of the impedance to be measured is output to the output terminal of the operational amplifier, whereby an extremely small impedance can be detected. In particular, when each impedance is a capacitive element, a minute capacitance of the order of several pF or fF or less can be detected.
In addition, since the non-inverting input terminal of the operational amplifier is connected to a predetermined potential and a direct current voltage is applied to the inverting input terminal via the resistor, the operational amplifier can operate stably and suppress noise included in the detection signal. In addition, since the whole detection circuit is operated by DC drive, an alternating current signal transmitter and the like are not required, so that simplification and integration of the circuit can be realized.
Further, since the capacitor is connected between the operational amplifier and the impedance converter, a problem that the S/N ratio is deteriorated by thermal noise from the resistor when the resistor is connected between the operational amplifier and the impedance converter does not occur.
Further, by disposing circuit elements connected to signal lines in close proximity to each other or disposing the impedance detection circuit and the impedance to be detected at adjacent positions, it is possible to eliminate the need for a shielded cable for connecting the above elements, a special circuit for eliminating parasitic capacitance generated in the cable, and the like.
Further, since a variation component corresponding to an impedance variation component of the impedance to be measured is generated in the output terminal of the operational amplifier, by extracting only the variation component of the output terminal, an amplification circuit suitable for a capacitive sensor, such as a microphone or the like, whose capacitance varies with a variation in a physical quantity can be obtained. In this way, for example, a microphone that detects sound with very high sensitivity can be realized.
Further, one end of the capacitor to be measured and the input terminal of the impedance converter may be connected to each other through a signal line covered with a shield member, and a protective voltage applying member may be added to apply a voltage having the same potential as that of the signal line to the shield member. Thus, the signal line is protected by the shield layer having the same potential, and the parasitic capacitance generated between the signal line and the shield layer can be eliminated, so that the minute capacitance can be detected with high accuracy.
As described above, according to the present invention, it is possible to accurately detect minute impedance and capacitance, and to realize a capacitance detection circuit suitable for miniaturization, and in particular, it is possible to improve the sound performance of a portable and small-sized voice communication device such as a mobile phone, and thus its practical value is very high.
Industrial applicability
The capacitance detection circuit of the present invention can be used as a detection circuit of a capacitance sensor, and particularly, can be used as a microphone device mounted in a small and lightweight device such as a mobile phone.
Claims (15)
1. An impedance detection circuit which outputs a detection signal corresponding to an impedance of an impedance to be detected,
the circuit comprises: an impedance converter having a high input impedance and a low output impedance; a capacitive first impedance element; a first operational amplifier; a dc voltage generator for applying a dc voltage to the first operational amplifier; a signal output terminal connected to an output of the first operational amplifier;
wherein one end of the impedance to be measured and one end of the first impedance element are connected to an input terminal of the impedance converter,
the first impedance element and the impedance transformer are included in a negative feedback loop of the first operational amplifier,
the impedance under test is disposed adjacent to the impedance detection circuit.
2. An impedance detection circuit which outputs a detection signal corresponding to an impedance of an impedance to be detected,
the circuit comprises: an impedance converter having a high input impedance and a low output impedance; a capacitive first impedance element; a first operational amplifier; a dc voltage generator for applying a dc voltage to the first operational amplifier; a signal output terminal connected to an output of the first operational amplifier;
wherein one end of the impedance to be measured and one end of the first impedance element are connected to an input terminal of the impedance converter,
the first impedance element and the impedance transformer are included in a negative feedback loop of the first operational amplifier,
the impedance to be measured is disposed in proximity to the first impedance element and the impedance transformer.
3. The capacitance detection circuit of claim 1 or 2, wherein the impedance under test is a capacitive impedance element.
4. The capacitance detection circuit of any one of claims 1-3, further comprising a resistive element connected in parallel with the first impedance element.
5. The impedance detection circuit of any one of claims 1 to 4, further comprising a second impedance element between the AC voltage generator and the first operational amplifier.
6. The impedance detection circuit of any one of claims 1 through 5,
one end of the impedance to be measured is connected to an input terminal of the impedance converter through a signal line covered with a shield member,
the impedance detection circuit further includes a protection voltage applying part for applying a given voltage to the shielding part.
7. The impedance detection circuit of claim 6, wherein the protection voltage application means has an output voltage of the direct current voltage generator as an input.
8. The impedance detection circuit of claim 6, wherein the protection voltage application component has as an input the output voltage of the impedance transformer.
9. The impedance detection circuit of any one of claims 1 to 8, wherein the impedance transformer is a voltage follower.
10. The impedance detection circuit according to any one of claims 1 to 8, wherein the impedance converter is a voltage amplification circuit including a second operational amplifier and having a voltage gain greater than 1.
11. The capacitance detection circuit of any one of claims 1-8, wherein the impedance converter comprises an input circuit comprised of a MOSFET and a second operational amplifier.
12. The capacitance detection circuit according to any one of claims 1 to 11, wherein the impedance to be detected is a capacitive sensor in which a temporal change in capacitance occurs, and the first impedance element is a capacitor.
13. The capacitance detection circuit of claim 12, wherein the impedance being measured is a capacitive microphone.
14. An impedance detection method for outputting a detection signal corresponding to an impedance of an impedance to be detected,
a capacitive first impedance element is connected between the output terminal of the operational amplifier and the input terminal of the impedance transformer,
a measured impedance is connected between the input terminal of the impedance converter and a given potential,
applying a direct-current voltage to one input terminal of the operational amplifier via a resistor and making the other input terminal a given potential,
outputting a voltage appearing on an output terminal of the operational amplifier as a detection signal,
the impedance to be measured and the impedance transformer and the first impedance element are closely connected.
15. The impedance detecting method according to claim 11, wherein, in the impedance detecting method,
one end of the impedance to be measured is connected to an input terminal of the impedance converter through a signal line covered with a shield member,
a given voltage is applied to the shielding member.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP270877/2001 | 2001-09-06 | ||
| JP2001270877 | 2001-09-06 |
Publications (2)
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| CN1842712A true CN1842712A (en) | 2006-10-04 |
| CN100454028C CN100454028C (en) | 2009-01-21 |
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| CNB028173929A Expired - Fee Related CN100454028C (en) | 2001-09-06 | 2002-09-06 | Impedance measuring circuit, its method, and capacitance measuring circuit |
Country Status (9)
| Country | Link |
|---|---|
| US (1) | US7005865B2 (en) |
| EP (1) | EP1426772B1 (en) |
| KR (1) | KR100654471B1 (en) |
| CN (1) | CN100454028C (en) |
| AT (1) | ATE389185T1 (en) |
| DE (1) | DE60225570T2 (en) |
| NO (1) | NO20032013L (en) |
| TW (1) | TWI221196B (en) |
| WO (1) | WO2003023420A1 (en) |
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| CN104685904A (en) * | 2013-01-08 | 2015-06-03 | 陈冰丽 | Condenser microphone and its impedance converter |
| CN105277786A (en) * | 2014-07-25 | 2016-01-27 | 南京瀚宇彩欣科技有限责任公司 | Method for detecting maximum impedance of sensor and detection device |
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| US8247883B2 (en) * | 2008-12-04 | 2012-08-21 | Palo Alto Research Center Incorporated | Printing shielded connections and circuits |
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| JP3454426B2 (en) * | 2000-07-10 | 2003-10-06 | 住友金属工業株式会社 | Impedance detection circuit and impedance detection method |
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-
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- 2002-09-05 TW TW091120338A patent/TWI221196B/en not_active IP Right Cessation
- 2002-09-06 KR KR1020047003319A patent/KR100654471B1/en not_active IP Right Cessation
- 2002-09-06 EP EP02767921A patent/EP1426772B1/en not_active Expired - Lifetime
- 2002-09-06 CN CNB028173929A patent/CN100454028C/en not_active Expired - Fee Related
- 2002-09-06 WO PCT/JP2002/009137 patent/WO2003023420A1/en active IP Right Grant
- 2002-09-06 DE DE60225570T patent/DE60225570T2/en not_active Expired - Lifetime
- 2002-09-06 US US10/488,679 patent/US7005865B2/en not_active Expired - Fee Related
- 2002-09-06 AT AT02767921T patent/ATE389185T1/en not_active IP Right Cessation
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2003
- 2003-05-05 NO NO20032013A patent/NO20032013L/en not_active Application Discontinuation
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| CN103777047A (en) * | 2012-10-23 | 2014-05-07 | 基思利仪器公司 | Impedance source ranging apparatus and method |
| CN103777047B (en) * | 2012-10-23 | 2019-09-10 | 基思利仪器公司 | Source of resistance range adjustment equipment and method |
| CN104685904B (en) * | 2013-01-08 | 2017-10-03 | 广东得胜电子有限公司 | Electret Condencer Microphone and its impedance transformer |
| CN104685904A (en) * | 2013-01-08 | 2015-06-03 | 陈冰丽 | Condenser microphone and its impedance converter |
| CN103344172B (en) * | 2013-07-09 | 2015-08-12 | 中国工程物理研究院总体工程研究所 | A kind of capacitance gage acquisition processing device and method reducing electromagnetic pulse interference |
| CN103344172A (en) * | 2013-07-09 | 2013-10-09 | 中国工程物理研究院总体工程研究所 | Capacitance micrometer acquisition processing device and method reducing electromagnetic pulse interference |
| CN105277786A (en) * | 2014-07-25 | 2016-01-27 | 南京瀚宇彩欣科技有限责任公司 | Method for detecting maximum impedance of sensor and detection device |
| CN107422192A (en) * | 2017-08-02 | 2017-12-01 | 广州市锐丰音响科技股份有限公司 | Audio frequency apparatus input impedance measuring method and device, measurement method of calibration and device |
| CN107422192B (en) * | 2017-08-02 | 2020-05-08 | 广州市锐丰音响科技股份有限公司 | Audio equipment input impedance measuring method and device and measurement and verification method and device |
| CN107274644A (en) * | 2017-08-11 | 2017-10-20 | 四川赛科安全技术有限公司 | A kind of method for detecting fire-fighting bus capacitive size |
| CN108693408A (en) * | 2018-08-22 | 2018-10-23 | 广东电网有限责任公司 | Ultracapacitor internal resistance detection method, device and computer readable storage medium |
| CN108693408B (en) * | 2018-08-22 | 2020-07-07 | 广东电网有限责任公司 | Supercapacitor internal resistance detection method and device and computer readable storage medium |
| CN114812618A (en) * | 2021-12-24 | 2022-07-29 | 中国科学院长春光学精密机械与物理研究所 | Frequency point noise suppression system |
Also Published As
| Publication number | Publication date |
|---|---|
| NO20032013D0 (en) | 2003-05-05 |
| ATE389185T1 (en) | 2008-03-15 |
| EP1426772A1 (en) | 2004-06-09 |
| EP1426772B1 (en) | 2008-03-12 |
| TWI221196B (en) | 2004-09-21 |
| US7005865B2 (en) | 2006-02-28 |
| EP1426772A4 (en) | 2005-08-03 |
| CN100454028C (en) | 2009-01-21 |
| DE60225570D1 (en) | 2008-04-24 |
| NO20032013L (en) | 2003-06-19 |
| KR100654471B1 (en) | 2006-12-05 |
| US20050035771A1 (en) | 2005-02-17 |
| KR20040040454A (en) | 2004-05-12 |
| DE60225570T2 (en) | 2009-04-23 |
| WO2003023420A1 (en) | 2003-03-20 |
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